It is usual in sewage treatment plants for a flocculant to be added to water which is subsequently centrifuged, e.g. by means of a decanter centrifuge, for removing the coagulated particulate contaminants from the water. The flocculating agents commonly used, such as polyelectrolyte flocculants, are expensive and make a significant contribution to the total water treatment costs. It is necessary for sufficient flocculant to be added to the effluent so that the water is clarified to the level required, but any more than the optimum amount of flocculant means unnecessary additional expense and hence uneconomic operation of the plant. It is known to monitor the quality of the clarified water by making turbidity measurements, and to adjust the rate of consumption of flocculating agent in accordance with the results obtained. While the theory behind this technique for controlling the water purification process is very sound, it has not been possible to implement it effectively because of the limitations imposed and by and suffered by commercially available turbidity meters. Although there are many turbidity meters of various types on the market, none have proved entirely satisfactory for monitoring a clarified liquid stream discharged from a decanter centrifuge in an effluent purification process, and only limited success has been achieved with such meters.
It is well known to measure the turbidity of a liquid by measuring the intensity or attenuation of light transmitted through a sample of the liquid. It has also been proposed to do so by measuring the light scattered and/or reflected from solid particles present in the liquid. In the former method a diminishing signal, which signal indicates the intensity or amount of transmitted light, denotes an increase in the suspended solid particles present, i.e. the turbidity of the liquid, whereas in the latter method an increase in the amount of light detected indicates more particles are present. If the light reflected from the particles is relied upon to provide a measurement, as proposed in GB 1281342, the device will have a very poor response unless the particles are white, or at least very light in colour, assuming visible light is used by the sensor. There is described in WO82/03460 a turbidity meter with a probe consisting of a bundle of optic fibres, some of which are used to emit light and others of which are used to detect the reflected light, but the device is specifically adapted for turbidity measurements on substantially opaque highly turbid liquids, e.g. to determine the butterfat content of milk, and therefore it utilises only a very small sample of liquid immediately in front of the probe.
The reasons why turbidity meters have so far failed to give satisfactory results in their practical application to effluent treatment as mentioned above are several and various. In some cases the range of measurement is wrong or is too narrow; blockage problems are sometimes experienced, particularly where liquid flow passages are small; lens contamination occurs necessitating frequent cleaning operations; or they simply do not give readings reliable enough to enable effective control over the flocculant consumption. One reason for unreliable results is that gas or air bubbles are detected as if they were solid particles. An affect frequently caused by over flocculation is aeration of the clarified water leaving the decanter centrifuge, with the result that numerous bubbles are present and the liquid has a tendency to foam. With bubbles being seen as if they were particles the reading from the turbidity meter can be very misleading to the extent that it suggests more rather than less flocculant needs to be added upstream of the centrifuge. Because of these difficulties it has become accepted practice in the art of waste water treatment to subject the clarified liquid to a de-aeration treatment before passing it to the turbidity meter. In addition, to alleviate the problems associated with precipitation on the lens and thereby reduce the frequency of cleaning operations, it is normal practice to dilute the de-aerated sample before admitting it to the turbidity meter.
From the point of view of making accurate measurements, dilution is disadvantageous since allowance must be made for the effect of the dilution on the actual turbidity of the initial sample. Added to that, both de-aeration and dilution are inconvenient because they are extra steps which must be performed in the water treatment process.
The present invention addresses the problems associated with known turbidity measuring equipment as explained above, and is based on the realisation that the presence of gas bubbles in the liquid sample can be taken advantage of to produce more reliable measurement of turbidity, at least in as much that greater sensitivity to fluctuations in turbidity is possible.